>>588130Sum 2 bottom left equations. Let's call resulting equation 2'

Sum 2 bottom right equations. Let's call resulting equation 3'

Now use 2' to express x in terms of z. Let's call resulting equation 2''

Substitute resulting x into 3'. Compute value of z.

Substitute computed value z into 2'' to compute x.

Now substitute x and z into any of original equations to compute y.